E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Strike while the Iron is Hot.
In French, Il faut battre le fer pendant quil est chaud. Either act while the impulse is still fervent, or do what you do at the right time. The metaphor is taken from a blacksmith working a piece of iron, say a horse-shoe, into shape. It must be struck while the iron is red-hot or it cannot be moulded into shape. Similar proverbs are Make hay while the sun shines. Take time by the forelock.